A late-Spring pulse thunderstorm event developed quickly during the afternoon and evening hours of June 2nd. A few severe, hail producing thunderstorms initially developed across northern Kentucky. Coverage then quickly increased closer to a boundary across central and southern Ohio. During a period from June 2nd at 18Z to June 3rd at 4Z, the WFO in Wilmington, Ohio (ILN) issued 35 severe thunderstorm warnings, 2 tornado warnings, and 1 flash flood warning. The event was highlighted by two long-tracked, rotating supercell thunderstorms which developed along the boundary and produced widespread large hail, wind damage, and a brief tornado touchdown.

A cold front sagged slowly southward on the night of June 1st and became nearly stationary along a line from Richmond, Indiana to Columbus, Ohio by June 2nd at 18Z. A weak low developed across the Missouri Valley through this period with a 1022 mb surface high centered across Lake Michigan. By the late afternoon on June 2nd, the frontal boundary remained located along and just south of the Interstate 70 corridor. An upper level jet max across the northern Great Lakes remained north of the Ohio Valley, resulting in a lack of upper level dynamics in this event. At the mid-levels, a westerly flow was predominant with no significant short wave apparent. At 850 mb, diffluence was noted along with abudant moisture across the Ohio Valley. At the surface, a broad pressure gradient was in place across the Ohio Valley with a tight theta-E gradient along the front. South of the front, a warm, moist air mass was present with temperatures in southern Ohio having warmed into the mid to upper 80s. Meanwhile, north of the front, temperatures were only in the lower 60s. Wind fields indicated low-level convergence along the front.

18Z HPC Surface Analysis

18Z RUC Upper Air and Surface Analysis

300 mb

500 mb

850 mb

Surface

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Mesoscale Analysis and Initialization

The Day 1 Storm Prediction Center (SPC) convective outlook issued at 6Z on June 2nd highlighted most of the CWA with a slight risk. Threat probabilities were: 15% hail, 15% wind, and 2% tornadoes. Their discussion highlighted modest mid level lapse rates, low-level moistening, and surface heating contributing to forecast moderate instability of 1000-2000 J/kg. Storms were expected to develop along the cold front, which had stalled as a boundary across central Ohio. Also, forecast effective bulk shear of 35-40 knots near the front was noted to be sufficient for the development of supercells and possibly linear convection. No widespread hail threat was mentioned for the area in the discussion.

Storm Prediction Center Graphics from (6Z June 2)

Day 1 Outlook

Tornado Probability

Large Hail Probability

Damaging Wind Probability

Click each image to link to the SPC Graphic

The 12Z ILN sounding indicated CAPE of 2215 J/kg with a precipitable water value of 1.06 inches. With the radiational
inversion at the surface eroding quickly in the morning hours, plenty of instability was available aloft across areas south of the boundary. By 18Z, LAPS data indicated surface-based CAPE values near 2500 J/kg. Both LAPS and RUC data indicated the best bulk 0-6km shear (30-40 knots) was near the boundary across central Ohio and into east-central Indiana. The tornado threat appeared to be minimal with 0-3km SRH values of 100-200 m2s-1 and 0-1km bulk shear of around 10 kts. Hail appeared to be the primary threat, with low-end strong CAPE values and web bulb zero heights of 10-11 kft. Scattered pulse convection developed across northern Kentucky around 18Z with the first severe thunderstorm warning issued at 1844Z. Around 19Z, with a moist, unstable air mass and strong instability, convection began to develop rapidly along and south of the boundary. The highest concern for supercell development was along the boundary with the best forcing in addition to both moderate CAPE and 0-6km bulk shear. After the initial development, additional severe thunderstorms developed along numerous outflow boundaries. Along with the development of numerous severe hail producing storms, two of the storms located along the boundary took rotational characteristics which allowed them to be long-tracked, hail producing storms. One of the storms had a confirmed tornado touchdown in Fairfield County. (For more information on the evolution of these two storms, see the Feature Following Zoom section below.)

18Z Mesoscale Analysis

MSAS Surface Tds and Winds

LAPS CAPE, CIN, and 0-6km Bulk Shear

LAPS CAPE, CIN, and 0-6km Bulk Shear

12Z ILN Sounding

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Feature Following Zoom Storm Analysis

As part of the AWIPS Operational Build 9 upgrade, a new feature was added which allows forecasters to track storms as they traverse across several counties or even for hundreds of miles. After the event, the Feature Following Zoom (FFZ) function on AWIPS allowed forecasters to analyze the evolution of two storms as they tracked 90 and 120 miles respectively. For the first time, forecasters can now zoom in closely to storms and can observe subtle storm-scale features throughout the track of the storm in real-time.

The Dayton-Hocking Storm:
This storm developed just west of Dayton and pulsed as it crossed the Montgomery-Greene county line, prompting its first severe thunderstorm warning at 3:15pm. At 3:17pm, the first report of hail with the storm was received north of Beavercreek, where 1 hail inch was reported. As the storm trekked eastward across Madison County, its outflow boundary moved south of the storm and interacted with another cell in Fayette County. As the parent storm moved into Pickaway County, it produced a microburst with several homes damaged in Darbyville. The cell which had developed off the initial outflow boundary of the parent storm continued to move northeast and merged with the main storm in eastern Pickaway County. With the cell-merger, the storm took on more supercellular characteristics north of Circleville, with a well-defined circulation. The storm developed a rear-flank downdraft in extreme southeast Fairfield county which caused microburst damage in the Stoutsville community. A short-lived hook echo is evident just south of Amanda, which is where a brief tornado touchdown was observed and confirmed with a storm survey. The storm finally began to weaken in Hocking county, where wind damage downed trees on several roads and one inch hail was reported. This storm tracked for nearly 90 miles across central Ohio and aside from the wind and tornado damage, hail as large as 2 inches in diameter accompanied by strong winds did significant damage to several homes and vehicles.

Dayton-Hocking Storm

KILN radar base reflectivity from 1850Z-2209Z (radar loop)

The vinyl siding of a home is severely damaged from 2" diameter hail in Pickaway county. (photo -- Jim Bingman)

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The Henry-Fayette Storm:
Originating just north of Indianapolis in eastern Marion County, this storm began to take shape in east-central Indiana. The first severe thunderstorm warning for this storm originated in Indianapolis' County Warning Area (CWA) in Henry County at 3:43pm. This storm produced hail which did vehicle damage in its early stages, and crossed into the Wilmington, OH CWA around 4:18pm. As the storm passed through Wayne County, Indiana, it was evident that it had become a rotating supercell, with a bounded weak echo region, mesocyclone aloft, and reflectivity core of over 70 dbZ. Golf ball sized hail was first reported with this storm in Centerville, Indiana. The storm began to demonstrate rotational characteristics in the lowest levels as it crossed into Preble County, Ohio, prompting a tornado warning. There were no reports of funnel clouds or tornadoes with this storm. However, the storm continued to drop golf ball size hail as it continued through Preble and Montgomery Counties, and there were even a few reports of hail as large ping pong ball size. As the storm moved into Warren County, the mesocyclone weakened, and the storm became more cold pool driven. Wind damage occurred across portions of Clinton County and the storm crossed directly over the National Weather Service office where a wind gust of 68 mph was recorded with dime size hail. A barn was partially damaged next to the office and there were several reports of trees down and some structural damage across the county. The storm finally weakened as its cold pool raced out ahead of it in southern Fayette County. The final report of hail with the storm occurred at 7:50pm, over 4 hours after warnings were first issued with the storm.

Henry-Fayette Storm

KILN radar base reflectivity from 1932Z-2358Z (radar loop)

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NSSL Rotational Tracks and Hail Swaths

The National Severe Storms Laboratory (NSSL) in Norman, OK has developed products and tools which can assist forecasters to make accurate and timely warnings decisions during severe events. The NSSL OnDemand Rotational Tracks and Hail Swath products have also proven useful in the post-storm analysis, verification, and storm survey for this event.

Rotational Tracks:
This KMZ product was uploaded to Google Earth after the event using the NSSL OnDemand request system. This product displays the velocity data from each radar through a Linear Least Squares Derivative (LLSD) filter which creates an azimuthal shear field. The 0-3km layer of the azimuthal shear is taken from each radar across the CONUS are then combined so that the maximum value at 250 m2 gridpoint is accumulated over time and then plotted. This product displayed the rotation in both the Henry-Fayette and the Dayton-Hocking storms. The best rotation was found just south of Amanda and north of Stoutsville. This data was used as guidance during the storm survey and it was found that the track correlated very well with the brief tornado touchdown south of Amanda. The rotational track went just north of Stoutsville, where it is believed the rearflank downdraft led to structural damage before initiating the descent of the tornado.

The Rotational Tracks product was also looked at during the May 30 tornado touchdown behind the NWS office in Wilmington. Unfortunately, the product appears cluttered nearby the radar and a rotational track with the storm could not be discerned. To view the May 30 product visit: May 30 Rotational Tracks.

NSSL MESH product showing large hail.

Large hail in Centerville, IN (photo -- Gary Locke).

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With the successful use of these products using Google Earth after this event, it has been found that both of these Warning Decision Support System -- Integrated Information (WDSS-II) products can be used in real-time as part of a Situational Awareness Display. The hail swaths can aid forecasters in estimating the historical hail size of a cell, whereas the rotational tracks can help forecasters get a feel for the length and extent of a circulation with a storm.
To view the MESH product from the event visit: June 2 Hail Swaths.

RUC Temperature Planes and theFour-Dimensional Stormcell Investigator

Advanced techniques used more recently by National Weather Service forecasters in storm interrogation include analyzing temperature plane reflectivity values and the Four-Dimensional Stormcell Investigator (FSI). These features allow forecasters to deeply interrogate features aloft which can contribute in the warning decision process.

The Four-Dimensional Stormcell Investigator:
FSI provides a multi-faceted cross-section of a storm, where forecasters can analyze storm reflectivity, velocity, storm-relative velocity, and spectrum width. Before FSI, forecasters could only interrogate a single slice of a storm through a cross-section. FSI allows its users to pan, zoom, and rotate the axis of the cross-section instantaneously. In both the Henry-Fayette and Dayton-Hocking figures below, FSI indicates echo overhang above the weak echo region with a Bounded Weak Echo Region (BWER) where the area of weak reflectivity noses into the overhang and reflectivity core of the storm. This is correlated with an updraft that will allow for hail and moisture to remain suspended aloft for an extended period of time. This, along with persistent rotation in a storm, can be indicators of large hail (2" diameter or greater).

FSI display from 2037Z Jun 2 of the Henry-Fayette storm (left) and from 2053Z of the Dayton-Hocking storm

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RUC236 Temperature Planes:
Along with the FFZ function mentioned previously, AWIPS Operational Build 9 also allows forecasters to interrogate storms using constant temperature surfaces from RUC236 data. This is a new method for analyzing the structure and strength of storms. Prior to this new feature, reflectivity cross sections and the all-tilts scanning strategy have been the primary methods for interrogating the reflectivity structure aloft in storms. Now, critical levels, such as the freezing level and levels within the hail growth zone (-10 to -30C) can be analyzed on a single plane. Ultimately, this strategy saves forecasters the extra step of determining the freezing and -20C height, which can vary across a County Warning Area (CWA).

The June 2nd severe weather outbreak proved to be an event where the best instability, wind shear, and forcing were in phase along and just south of a frontal boundary. This served to develop storms with intense updrafts, two of which exhibited strong rotation.

The Storm Prediction highlighted the threat for severe weather well in advance, starting on May 31, with its Day 3 Outlook. The Ohio Valley continued to be highlighted into the Day 1 Outlook for severe weather, although the moderate instability and rotating super cells contributed to more of a hail producing event than first forecast, the storms did transition to become more cold pool driven, producing wind damage toward the end of their tracks.

Several new methods and tools were proven to be effective methods for storm interrogation. The Feature Following Zoom function was used for the first time at the office in post-storm analysis and was found to be very effective in picking up subtle storm-scale features as it tracked the storms 90 and 120 mile paths. The use of FSI and temperature planes were used to display of BWERs and intense reflectivity cores aloft. This data correlated well with the locations which reported the largest hail in both the Dayton-Hocking and Wayne-Fayette storms.

The NSSL rotational tracks and MESH products were found to be very effective during the post storm analysis in determining where the largest hail and best rotation with storms occurred. The rotational tracks product was used during a storm survey and the best rotation correlated well with the brief tornado touchdown in Amanda. This has led to the suggested use of the products in the WFO Wilmington, OH Google Earth Situational Awareness Display.